The present invention pertains to the art of gas turbines and, more particularly, to a gas turbine endcover assembly including a plurality of selectively operable integrated control valves.
In general, gas turbine engines combust a fuel/air mixture in a number of combustion chambers to release heat energy that is channeled to a turbine. A central fuel or gas supply is linked to each of the combustion chambers. The central supply is operated to deliver an amount of fuel through a supply line that is linked to a common manifold which supplies all of the combustion chambers. The fuel is mixed with air and ignited to form the high temperature gas stream. The turbine converts thermal energy from the high temperature gas stream to mechanical energy that rotates a turbine shaft. The output of the turbine may be used in a variety of applications such as, for example, powering an electrical generator.
Several byproducts of combustion, such as nitrous oxide (NOx) carbon monoxide (CO), and unburned hydrocarbons (UHC) are subject to federal and state regulatory limitations. NOx is produced by the oxidation of nitrogen brought in with atmospheric air and is exponentially depending on flame temperature above 2500° F. (1371° C.). In order to maintain NOx emissions within emission compliance, flame temperatures must be maintained below 3000° F. (1649° C.). One method employed to control NOx emission is the injection of inerts, e.g., steam, water, nitrogen into the combustor. The injection of inerts results in a lean mixture and much lower NOx. However, large quantities of very clean steam or water are required, and in some areas, the cost of water/steam can exceed the cost of fuel. The injection of water also has a negative impact on emissions often resulting in an increase in the production of CO and UHCs.
Operation at low bulk fuel/air ratio, near a lean extinction limit, is particularly difficult at reduced load. That is, during off-peak hours operating a generator at full output is not practical. Any energy produced over demand that is not otherwise sold is wasted. Accordingly, balancing low output with lean operation while mainlining emissions compliance is difficult. In order to address this problem the turbine is operated at a piloted-premix in which some 10 to 20% of the fuel is injected directly into the reaction zone and bums as a high temperature diffusion flame. This provides good stability and combustion efficiency, but NOx levels are out-of-compliance. Thus, the turbine is alternately operated in an out of compliance state and in compliances state to maintain average emissions output in compliance.
In addition to the above, restarting a gas turbine combined cycle generator that was shut off is a lengthy process that may take an hour or more before full output is achieved. This lost time can be quite costly for an energy producer. Moreover, a generator that is shut off is not available in the event that additional output is unexpectedly needed during a low demand period. In addition, starting and stopping a generator impacts the durability and life of power system components. Frequent starts and stops will have a detrimental impact on engine reliability and trigger a need for more frequent maintenance cycles thus increasing operational and maintenance costs.
Given the drawbacks, associated with stopping the gas or combined cycle turbine, energy producers prefer to turn down or park the engine during off peak hours to minimize the fuel burned while maintaining the ability to respond to an unplanned increase in load. Parking the turbine engine at a point that allows a quick return to full power, while also remaining emission compliant, is a difficult balancing act for the reasons outlined above. Therefore, when parking a turbine, the engine is operated at a specific part load condition with brief periods of out-of-compliance operation. While effective at maintaining an engine within emission compliance, achievable part load conditions are still high, in the range of 40% of normal output, and thus can represent substantial operational inefficiencies.
In addition to the above, an important over-arching constraint that represents a significant initial barrier and steady, day-to-day struggle in successfully addressing all emissions, reliability and operational flexibility requirements of a turbine engine is the variation inherent in any ‘real-world’ power plant context. Performance of a lean, premixed combustion system may be impacted by minute changes in external variables. Variation in individual fuel circuit flow (fractions of 1% of the total), night/day and seasonal variations in ambient temperature and relative humidity, site location and elevation, and incremental (a few percent by volume) changes in fuel gas composition, as well as power system load will all impact combustion system performance.
Furthermore, internal variables, such as the chamber-to-chamber variation in air and fuel flow as a result of dimensional differences in system components, and shifts in secondary flows as a result of variations in upstream and downstream turbomachinery also impact combustion system performance. Further, these internal system variables will change with the age and condition of the parts as dictated by the number of hours and cycles and the specific operating ‘mission’ or path from first-fire to present day. Thus, at present, modem gas engines require multiple fuel circuits for simultaneous control of exhaust emissions and flexible reliable operation over a wide ranges of load, ambient conditions and fuel gas compositions.